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Author Topic: Did Chris make a mistake?  (Read 2157 times)

Offline Eric A. Taylor

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Did Chris make a mistake?
« on: 14/03/2010 02:54:25 »
On the podcast dated Feb, 23 Chris explained that the reason it's colder at higher altitude is that the air has less pressure.

  He explained that as air drops in pressure it loses energy and becomes cooler. This is true only in CHANGE of pressure. As the air at high altitude hasn't changed pressure much this reasoning is incorrect.

  What's really happening is that air is transparent to most of the light given off by the sun and does not absorb much energy. This is why the air looks mostly clear. The energy that IS absorbed (like high energy UV) is absorbed at very high altitudes (above 80,000 feet. The air this high is VERY hot but so thin "hot" and "cold" no longer have the same meaning as they do in the ground). The air is not heated by the sun. It's heated by the ground. This is why coastal towns near cold oceans (Like San Fransisco) tend to be chilly. I have seen for myself days when it's 60 degrees and foggy in Berkley, Cal. and 80 degrees and sunny in Ortinda, which is just on the other side of a mountain through a short tunnel (About 2 or 3 minutes driving if traffic isn't horrible).

  If Chris' explanation were correct then high altitude planes, like Western Oregon would always be cold.  Pendelton, Oregon (near Idaho) can see summer temperatures over 100.

  You guys do a great show but please be a little more careful with your explanations.


 

Offline chris

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Did Chris make a mistake?
« Reply #1 on: 14/03/2010 09:47:49 »
Dear Eric

Thanks for highlighting this point but no, I did not make a mistake: it is correct that expansion cools air at high altitudes.

You rightly point out that the atmosphere is relatively transparent to IR - infrared - (heat) and therefore not much is absorbed by the air at altitude as sunlight makes its way towards the Earth's surface.

The land surface, on the other hand, absorbs the energy and re-radiates it, including warming the air close to the ground. Thermometers measure air temperature, and hence we know the air is being warmed like this at ground level.

This warm air rises (creating winds in the process as evidence of this occurring) and as it does so it feels less pressure and hence expands. Thus, it notionally "does some work", reducing the kinetic energy of the particles (in other words the temperature).

Therefore, air at high altitude becomes very cold, so the conditions you experience at high altitude are also very cold. If you are airborne, there's no land surface nearby to warm the air and so the only contact you have is with very cold air.

The effect is no different to the "frisson" experienced when spraying a deodorant aerosol in your armpit!

In reappraising the answer I gave, I agree, however, that maybe I could have made more mention of the relative transparency of air to IR, and to the fact that I was referring to hot air rising from the planet's surface. I actually say "so as the gas in the atmosphere rises". I agree some extra information here might have made the answer clearer.

However, I also think that part of the problem is the definition you are using of "high altitude". Compared with altitudes higher than the top of Everest, which is what I was discussing in answering the question, Oregon is not at high altitude!

Here's a link to the original answer given in the show, if anyone is interested in following this up:

http://www.thenakedscientists.com/HTML/content/questions/question/2565/

We've also addressed this question before:

http://www.thenakedscientists.com/HTML/content/questions/question/2458/

Chris
 

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Did Chris make a mistake?
« Reply #1 on: 14/03/2010 09:47:49 »

 

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